An integrated circuit (IC) tester characterizes the electrical operation of a device under test (DUT) by applying a test voltage to each of the pins on the DUT and measuring each resulting current. Typically, an IC tester selects one of several measure resistors, applies a stimulus to a pin of the DUT using an input driver, and measures the current response of the DUT through the related measure resistor. Each measure resistor corresponds to a specific current range and measurement accuracy is proportional to the precision of the selected resistor.
Currently available IC testers, such as the A580 Series by Teradyne, Inc. and the Synchromaster HPC by LTX Corporation, typically contain discrete electrical elements (including discrete measure resistors and discrete relays for switching between the measure resistors) mounted on printed circuit boards and require 3 boards for every 8 DUT pins. The maximum allowable number of pins of the DUT is limited by the number of boards that may be positioned near the DUT. Furthermore, discrete elements tend to be either expensive or of relatively slow performance. Incorporating the functionality of many discrete elements into a few IC components, which may subsequently be mounted on a single printed circuit board, would increase the maximum allowable number of pins of the DUT, would reduce the number of discrete measure resistors required, and would improve the overall performance of the IC tester by using integrated circuitry.
In accordance with an illustrated preferred embodiment of the present invention, an IC tester uses diffusion resistors instead of discrete resistors for the measure resistors. Diffusion resistors possess non-ideal characteristics, such as non-linear resistance with voltage bias conditions and inaccuracy of resistor values. To minimize the non-linear characteristics, each measure resistor contains precision integrated resistors fabricated on a unitary substrate as a dual path device. The dual paths, biasing and measuring, provide a means for calibrating the precision of each diffusion resistor. The biasing path contains two split diffusion resistors of equal impedance in series while the measuring path is a single diffusion resistor having uniform resistivity. The mid-points of each path and the shared island tub are set to the same potential. Thus, a Taylor series expansion for a curved line is applied to the two endpoints of the measuring path resulting in a diffusion resistor having improved linearity. These precision integrated resistors when used with design techniques such as matching characteristics, thermal layout, leakage cancellation schemes, and linearization of the resistors produces a precision current measuring system operable over a wide range of conditions. Leakage current is reduced from the nanoAmpere (nA) range to the picoAmpere (pA) range since the mid-points and the island tub are at zero potential with respect to one another, thereby making the resistance of the measure resistors more precise.
In accordance with another illustrated preferred embodiment of the present invention, an IC tester uses diode switches instead of discrete relays to switch between the measure resistors. Leakage current, on the order of nanoamperes from a switch that is open or disabled, is dramatically reduced by reverse biasing the diodes in each diode switch about the switch's diode bridge output node by an equal amount so that the summed current at the output node is almost zero.
In accordance with another illustrated preferred embodiment of the present invention, an IC tester includes voltage spike suppression to minimize the effects on the DUT of voltage spikes caused by the fast response of the diode switches as the IC tester steps through the current measurement ranges. This voltage spike may be large enough to jeopardize the sensitive circuitry on the DUT itself. A spike suppression switch in the negative feedback path of the input driver overrides the feedback circuit during switching to reduce the spike to a safer level.